The vertebrates of the Pirabas Formation (Eo-Miocene) are a very diverse group, represented by modern forms that have representatives / descendants in the present ecosystems. Sharks, rays, bony fishes and sirenians are abundant in the formation and, consequently, have received more attention compared to the less frequent crocodilians and chelonians. Despite the relatively significant amount of chelonian material, only recently has the group received a taxonomic determination effort. This is due to the fact that chelonians are represented mainly by osteoderms, containing few features of diagnostic value. In this work we continued the taxonomic studies of chelonians from the Pirabas Formation, aiming to complement the data already obtained, in order to refine the knowledge of vertebrate paleobiodiversity, as well as to contribute to the paleobiogeography of the South American Tertiary marine chelonians. The material analyzed consists of osteoderms deposited in the Paleontological Collection of the Museu Paraense Emílio Goeldi (MPEG), which were examined for type and general morphology, and compared with recent chelonian and fossil material cited in the literature. The results confirm the presence of at least two taxa in the formation: one representative of the suborder Cryptodira, belonging to the present marine chelonian family Cheloniidae; and one of the suborder Pleurodira, hyperfamily Pelomedusoides, which may belong to either the Podocnemididae or the Bothremydidae family, both of which have marine Miocene fossil representatives. The taxonomic data are consistent with the depositional environment types proposed for the Pirabas Formation and Caribbean Province. However, despite the analysis of additional material, poor preservation precludes progress in determining taxa at a more specific level.
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